Cohen–Sutherland algorithm

Question

I was reading Cohen–Sutherland algorithm on Wikipedia and I am confused in these two parts,

int ComputeOutCode(double x, double y) {
    ...
    if (x < xmin)       code |= LEFT;
    else if (x > xmax)  code |= RIGHT;
    if (y < ymin)       code |= BOTTOM;
    else if (y > ymax)  code |= TOP;
    ...
}

and

if (outcodeOut & TOP) {           // point is above the clip rectangle
    x = x0 + (x1 - x0) * (ymax - y0) / (y1 - y0);
    y = ymax;
} else if (outcodeOut & BOTTOM) { // point is below the clip rectangle
    x = x0 + (x1 - x0) * (ymin - y0) / (y1 - y0);
    y = ymin;
} else if (outcodeOut & RIGHT) {  // point is to the right of clip rectangle
    y = y0 + (y1 - y0) * (xmax - x0) / (x1 - x0);
    x = xmax;
} else if (outcodeOut & LEFT) {   // point is to the left of clip rectangle
    y = y0 + (y1 - y0) * (xmin - x0) / (x1 - x0);
    x = xmin;
}

What I am confused is, How does it handle points who are top-left, top right, etc. It just handles top, left, right and bottom.

Why use | in code |= LEFT; and outcodeOut & TOP in if-else below it? I know that these two somehow handle all 8 possible cases but I don't know how.

So, slipping into coaching mode: what do you understand |= does? — andy256
– andy256, Commented Dec 8, 2013 at 20:23

Sebastian Negraszus · Accepted Answer · 2013-12-08 23:15:56Z

3

code is a bit field which uses bitwise boolean operators to store whether certain flags are set or not.

code |= LEFT; is short for code = code | LEFT; and sets the LEFT flag. outcodeOut & LEFT is an expression that is only true (!=0) when the LEFT flag is set.

The shown excerpt does indeed not handle all (16) possible cases at once, but it's part of a larger loop that may run multiple times. So it's done in several iterations.

answered Dec 8, 2013 at 23:15

Sebastian Negraszus

3711 silver badge12 bronze badges

Now I am confused how line (200,200)->(900,850) is clipped for clip rectangle (100,100) (800,800). If we use outcodeOut&RIGHT block, it'll get solved. But the algorithm will use TOP block then RIGHT block, but its still solved somehow.

Sourabh
– Sourabh

2013-12-10 12:11:51 +00:00
Commented Dec 10, 2013 at 12:11

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